Edit: Completed the update, I don't see any new additions or improvements. Could be just bug fixes or could be the official GDR3. I guess it would be wise to just wait till WPCentral releases an article or Microsoft releases a change log.

I just randomly checked for updates on my AT&T Lumia 920 (developer unlocked on the GDR3 update), and I have two updates pending. I didn't get to take screenshots, but I will once the updates are finished installing. These were Microsoft updates, not Nokia ones.

EDIT: My version is now 10521.155. Don't notice any changes right now, but still nice to receive updates.

I had GDR3 before these updates were prompting me after checking. My version is now Microsoft Update 8.0.10521.155, while the latest on the Windows Phone update history page is still "OS version: 8.0.10501.127 or 8.0.10512.142*"....